FIG. 8 shows a conventional liquid-encapsulated bushing B. As is apparent from FIG. 8, the liquid-encapsulated bushing B includes an outer tube 04 which integrally has a ring 01, two stoppers 02 and 02 and partition members 03 and 03. An inner tube 07 integrally has a support shaft 05 and a collar 06. A bushing rubber 08 and rubber stoppers 09 and 09 are disposed between the outer tube 04 and the inner tube 07. Four liquid chambers R.sub.FU, R.sub.FL, R.sub.RU and R.sub.RL are defined between the bushing rubber 08 and the rubber stoppers 09 and 09. The two upper and lower front liquid chambers R.sub.FU and R.sub.FL, which are positioned on a front side, communicate with the two upper and lower rear liquid chambers R.sub.RU and R.sub.RL, which are positioned on a rear side, through first communication passages O.sub.1 and O.sub.1, respectively. Communication between the front liquid chambers R.sub.FU and R.sub.FL and communication between the rear liquid chambers R.sub.RU and R.sub.RL are provided through second communication passages O.sub.2 and O.sub.2, respectively.
If a low-frequency load of approximately 15 Hz is input in forward and rearward directions (an X-X' direction) and the outer tube 04 and the inner tube 07 move relative to each other, the bushing rubber 08, which connects the outer tube 04 and the inner tube 07, is deformed. Furthermore, the volumes of either the front liquid chambers R.sub.FU and R.sub.FL or the rear liquid chambers R.sub.RU and R.sub.RL are enlarged, while the other volumes are reduced. Thus, a liquid flows back and forth between the front liquid chambers R.sub.FU and R.sub.FL and the rear liquid chambers R.sub.RU and R.sub.RL through the first communication passages O.sub.1 and O.sub.1, respectively, whereby the low-frequency load is damped. If a medium-frequency load of approximately 80-250 Hz is input in upward and downward directions (a Z-Z' direction) and the outer tube 04 and the inner tube 07 move relative to each other, the volumes of either the upper liquid chambers R.sub.FU and R.sub.RU or the lower liquid chambers R.sub.FL and R.sub.RL are enlarged, while the other volumes are reduced. Thus, the liquid flows back and forth between the upper liquid chambers R.sub.FU and R.sub.RU and the lower liquid chambers R.sub.FL and R.sub.RL through the second communication passages O.sub.2 and O.sub.2, respectively, whereby the medium-frequency load is damped.
FIG. 9 shows a variation characteristic (shown by a solid line) of a dynamic spring constant due to a variation in the frequency of the upward and downward loads (the Z-Z' direction) which are applied to the liquid-encapsulated bushing B having the above-described structure. As compared with a variation characteristic (shown by a dotted line) of a bushing having no liquid chambers, the dynamic spring constant is lowered in the region of input frequencies of 80-230 Hz, whereby road noise can be reduced. However, in the conventional liquid-encapsulated bushing B, since the lowering of the dynamic spring constant is insufficient in the air column resonance region (220-250 Hz) of a tire which is contained in road noise components, a further lowering of the dynamic spring constant in this region has been desired. Although it is also possible to lower the dynamic spring constant by increasing the hardness of the bushing rubber 08, this method lowers riding comfort.